1. Field of the Invention
This invention relates to a liquid display device with an auxiliary capacitor corresponding to a pixel electrode.
More specifically, the invention relates to the structure of the auxiliary capacitor.
2. Description of the Related Art
Liquid crystal display devices have been widely used as display devices for TV sets and graphics display devices due to their advantageous features of thin structure and low power consumption. Because an active matrix type liquid crystal display device using thin film transistors (referred to as TFTs) as switching devices has the characteristics of high speed response and high precision, the liquid crystal display device is attractive for high picture quality, large size, and color pictures.
The liquid display device of such an active matrix type is generally composed of an array substrate, a counter substrate, and a liquid crystal layer held between these two substrates. The array substrate has a plurality of pixel electrodes each of them connected to a switching device (such as a TFT). On the other hand, the counter substrate has a counter electrode.
Such a liquid crystal display device as described above displays a picture by the capacitor made of the pixel electrode, the counter electrode, and the liquid crystal which is held between these electrodes.
Furthermore, in order to prevent the liquid crystal capacitor from discharging, an auxiliary capacitor is connected to the capacitor stored in the liquid crystal with parallel circuit relations.
One of the structures of this auxiliary capacitor is disclosed in Japanese Unexamined Patent Publication Hei 7-20494. This auxiliary capacitor comprises a lower electrode made of semiconductor material, an upper electrode made of metal material, and an insulation film held between the lower electrode and the upper electrode, and is called a MOS structure.
The length of the channel is important for such a MOS capacitor. If the length of the channel is too long, the resistance of the lower electrode is high, which causes a slow speed of response. As a result, the display quality is remarkably deteriorated.
Furthermore, if the semiconductor is made of polycrystalline silicon, the semiconductor layer needs to be hydrogenated. An object of the hydrogenation is to terminate dangling bonds in the semiconductor. Because the hydrogenation process is generally carried out after forming the insulating film and the upper electrode on the semiconductor, the upper electrode prevents hydrogen from getting into the semiconductor. In other words, the hydrogenation starts at the region not covered by the upper electrode, and then spreads to the region covered by the upper electrode. As a result, hydrogenating the whole region of the semiconductor takes a very long time.
If the time of hydrogenation is not enough, the region covered by the upper electrode, especially in the vicinity of the center region of the semiconductor, has many dangling bonds which causes the problem that the semiconductor does not function as the auxiliary capacitor, thus causing the deterioration of the display characteristic.